Download - Design of lower limb exoskeleton
Design of Exoskeleton for Paraplegic Patients
Alok Bharadwaj | Aditya S. N | Anirudh V KaushikR V College of Engineering
Bangalore
Overview
• Kinematics• Dynamics• Design• Analysis• Controlling• Design Features – Comfort level,– Manufacturability, – Compactness and structural stability.
Kinematics of Mechanism
1200
1200
Kinematics of Mechanism
• Hip (Passive joint)
Kinematics of Mechanism
• Hip and Knee joint• Use of mechanical
stoppers to limit the angular movement of limbs.
Kinematics of Mechanism
• Foot joint (Ball and Socket)
Dynamics of Mechanism
Assumptions
Mass of all links = 4kgMass of all motors = 4kgMass of battery pack = 2 kg Weight of human considered = 1000N
Leg portion Length Mass Weight
L1 450mm 5kg 50N
L2 500mm 3kg 30N
Dynamics of Mechanism
• Case 1 - Standing up
Figure A Figure B
Dynamics of Mechanism
• Standing up Torque required by motor at M2
= (1000 N) X (450 mm) = 450000 Nmm
However, as the motion begins, the position of upper body keeps changing. Thus, due to change of Center of gravity, required torque is much lower progressively.
Also, the crutches take up most of the body weight as the person can easily bend upper body to stand up.
This value is rounded off to 50000 Nmm shared by each leg. Thus, torque at each motor M2 is 25 Nm.
Dynamics of Mechanism
• Case 2 - Climbing stairsTorque required by knee motor= (1100 N) X (335 mm)=368500 Nmm
After considering the load taken up by using crutches and bending forward in order to climb stairs, this value is considerably lesser. (Assumed to be 50 Nm for each knee motor in each leg)
Thus, the maximum torque at knee considering previous case too becomes 50 Nm.
Dynamics of Mechanism
• Case 3 – Raising leg (maximum extent)
Torque required by motor at hip= (110 N) X (450 mm)=49500 Nmm
Thus, the maximum torque required at the hip joint is around 50 Nm at each motor of the hip joint.
Design
• Frame
Design
• Thigh Link
Design
• Shin Link
Design
• Leg
Design
• Complete Assembly – Ez Walk
Design
• Draft
Design
• Choosing the Battery:– For given maximum load @ 20rpm, the torque
required is 50Nm.– Required power = 100W per motor at 24V DC– Total max power input = 400W for motors– Overall power output = 500W – Battery assumed to last for two hours– Capacity needed 40000mAh– Weight of LiPo batteries ~ 3.5Kg
Analysis
• Material Used: Al 6061– Ultimate strength: 310Mpa– Yield strength: 276Mpa– Shear strength: 207Mpa– FOS considered: 3
• Failure mechanism: Von Mises
Analysis
• Critical Link: Thigh link
Analysis
• Critical section
Controlling the mechanism
Initiating walking
• Angle measurement using strain gauge measures the angle of upper body with respect to ground.
• Length of step proportional to step length.
Use of Ultrasonic Sensors
• 2 sensors are used in each leg – one at hip strap and one at foot
• Hip sensor – senses variation in the terrain
• Foot sensor – senses the vertical distance to the ground
Different cases requiring control
Walking
Different cases requiring control
Climbing Stairs UP
Different cases requiring control
Climbing Stairs DOWN
Safety Feature
• Crutches are provided to give proper balance when the person bends forward
• While climbing down, if the person leans forward (variation in strain gauge), the mechanism is turned off preventing his fall.
Advantages of Design
Comfort• Use of straps for even distribution of load• Weight evenly distributed on both sides• Weight of mechanism not being transferred to
the body.
Advantages of Design
Manufacturability• Links used and Aluminum 6061 are readily available
in the market.• Assembly is simple. Most assembly is just through
press fit. Fasteners are required only at a few places.
• No requirement of a CNC for manufacturing of any component
• Ball socket joints can be readily bought off the shelf
Advantages of Design
Weight• Aluminum has a very low density making the
mechanism very light• Evenly distributed loads at all locations. No point
loads.• Weight evenly distributed b/w both legs resulting
in no lateral CG offset. This makes the mechanism easy to use
• Low weight motors
Advantages of Design
Special features• Adjustable links• Portability – ease of assembly. Links can be
press fitted into each other. Very less use of fasteners.